Systems exist for the remote measurement of distance using optical methods. Such systems use methods include time of flight measurements, based on determining the difference in time between transmitting a pulse of light into the field, receiving the reflected wave at a detector, and measuring the time delay between the transmitted and the reflected signal. Knowing the wave propagation velocity, the distance of the target that reflected the light is calculated
There are several methods by which this delay time can be measured optically. Firstly, a simple transit time-of-flight measurement can be made for a single pulse transmitted to the target. Other methods are based on methods conventionally used in LIDAR technology. According to one method, the phase, difference is measured between a transmitted repetitive wave, such as a sinusoidal wave, and the wave received by reflection from the target point. The distance is a function of the wavelength of the light and of the phase difference between transmitted and reflected waves. Alternatively, the transmitted light can have a linear frequency chirp applied to it, or be streamed data and the difference in frequency between the transmitted and the received waves provides a measure of the distance from which the light was reflected from the object being ranged. Additionally, there exist methods whereby a predetermined coding scheme is applied to the transmitted light beam, and the transit time determined by the change in the code detected in the received reflected beam.
Such methods are very effective for measurement of the range to a single point remote from the rangefinder. However, the extension of these methods to rangefinding a complete line or even a complete 2-dimensional area involves hardware limitations which makes the conventional methods used for single point rangefinding difficult to apply for area rangefinding, which is essentially equivalent to 3-dimensional area mapping. To illustrate the magnitude of this problem, the example of terrain mapping is considered, such as would be performed in aerial mapping. In such a requirement, a surface profile may need to be mapped with an accuracy of several centimeters at a distance of hundreds of meters or more, by a moving aerial platform.
The direct frequencies of the optical spectrum involve very short wavelengths and very high frequencies, measurements which are complex and slow, such that ranging measurements are almost never made at the carrier optical frequencies. A common practice is to amplitude modulate the optical wave at a more manageable frequency, in order to lengthen the effective wavelength of the detection envelope and to lower the frequencies to be measured. Using the phase delay measurement method as an example, if the system is capable of making a phase measurement with an accuracy of 1/100 cycle, and it is required to make a distance measurement with an accuracy of 0.5 cm (i.e. 1 cm for the round-trip distance), the effective wavelength required will be 1 m, corresponding to an effective frequency of the amplitude modulated wave of 300 MHz. The amplitude modulation of the transmitted wave can be performed electrically in the laser source itself, or opto-electrically by means of a modulator at the output of the laser source in the transmitted optical path. Such a frequency range is readily measureable for individual pixel measurements, but becomes much more difficult when a detector array must be read, involving multi-measurements of large numbers of pixels in parallel. Signal detection and processing techniques currently available would have difficulty in performing such a task at acceptable real time speeds on a large imaging line or array. To illustrate the processing speeds required, if it is desired to determine the range to within 1/100 part of the wavelength of the amplitude modulated wave, then each cycle of the 300 MHZ modulated light needs to be sampled 100 times, leading to a sampling frequency of 30 GHz. When this has to be performed for a large number of parallel pixels, the processing speeds required are enormous, and difficult to achieve currently.
There therefore exists a need for a practical system and method for enabling effective three dimensional mapping in real time with high resolution, which overcomes at least some of the disadvantages of prior art systems and methods.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.